Multimode interference (MMI) devices, such as MMI couplers, are important integrated optical components for optical signal processing and routing. MMI devices typically utilize direct coupling in which the input waveguide is in contact with one or more output waveguides. This is in contrast to indirect coupling which relies upon evanescent field coupling through waveguides which are in close proximity to each other.
MMI devices may be used for beam splitting, combining and coupling and contain one or more input waveguides and one more output waveguides. Input and output waveguides are connected to a central multimode waveguide region. An MMI device of particular interest consists of one input waveguide and two or more output waveguides. The physical characteristics of the coupling and multimode waveguide regions are selected such that modal dispersion within the central multimode waveguide region provides for a single beam of light input into the first coupling waveguide to be split into the two or more second coupling waveguides. Operated in reverse, the device may function as a beam combiner.
MMI devices are subject to various losses associated with back reflections and other considerations due to mode mismatch and device manufacturing limitations. The splitting ratio of MMI devices, which is the ratio of power emerging from the output ports, is difficult to tune due to the design fabrication intolerances. Thus, there is a need to improve losses associated with MMI devices while obtaining desired splitting ratios.